Optical transmission, in which an information signal is modulated onto an optical carrier, is widely employed in modern communications systems. In particular, wide area communications networks employ long-haul transmission links using single-mode optical fibres for the transmission of digital information at very high bit rates, using one or more optical carriers, or wavelengths, over each fibre. With the advent of practical optical amplifiers, the maximum distance over which data may be transmitted in single-mode optical fibres before some form of regeneration is required is substantially limited by dispersion and nonlinear processes. In particular, the impact of chromatic dispersion increases rapidly as the bit rate of optical data channels is increased. Methods and apparatus capable of compensating chromatic dispersion, as well as other processes such as polarisation mode dispersion (PMD) and optical nonlinearities, are thus of vital importance in the deployment of modern communications systems.
While various techniques for performing compensation within the optical domain have previously been developed, there has more recently been great interest in the use of electronic compensation techniques, in part due to the greater capability for electronic systems (including digital- and software-based systems) to be adapted and/or reconfigured automatically in accordance with varying requirements, eg within different systems, different parts of a single system, and/or over time. By contrast, many optical techniques, and corresponding devices and apparatus, are not readily reconfigurable in the field.
One approach which enables highly reconfigurable electronic compensation for chromatic dispersion, and other effects of optical transmission, is disclosed in international application publication no. WO 2007/041799. In particular, there is described therein methods and apparatus which employ optical single sideband (OSSB) transmission, in combination with conventional direct detection (DD) techniques, to enable the use of frequency domain equalisation at the receiver, most preferably via orthogonal frequency division multiplexing (OFDM), for electronic dispersion compensation. This approach enables numerous advantages to be realised, including the use of relatively low-cost direct detection optical receivers, a high degree of scalability, good spectral efficiency, improved tolerance to dispersion, and simplified equalisation of the effects of chromatic dispersion in particular, without a corresponding scaling in the cost and/or complexity of apparatus at the transmitting or receiving ends.
However, it is well-known that further improvements in received signal quality, as measured by the signal-to-noise ratio (SNR) may be obtained, at least in principle, by using coherent detection techniques. In practice, however, coherent detection requires increased complexity and cost both in the components deployed within transmission links, and in ongoing operation and maintenance of the system.
It is therefore desirable to develop methods and apparatus that are capable of providing improvements in received signal quality (ie SNR) which may approach those available using coherent detection techniques, while retaining the benefits (eg relatively lower cost and greater simplicity) associated with commonly deployed direct detection receivers. The present invention seeks to provide such methods and apparatus.